Isn't that a great little write up about CSX3193!
CSX3193 History
I've never seen a photo of an Arntz beside 3193, but if you get a picture of a car at the correct angle it seems that there are way more similarities than differences in the general shape of the car. For what it's worth, having owned an Arntz and a Contemporary, I personally much prefer the shape of the Arntz.
Shape aside, the thickness of Arntz bodies changed. "Donunder" is a forum member here and he has an incredibly thick body. Around the wheel arches, over the fender tops, the trunk, hood...they are all nearly TWICE as thick as the Arntz I owned until recently (now owned by "Leroy17" here on the forum). The thick body might as well have been made of bullet proof steel. It makes the car more of a tank than a light weight sports car! OK...Maybe not that extreme, but the body will outlive civilisation before it encounters any stress cracks...
Donunder's thick body is the shape that includes a cutout of the cowl section that is then incorporated into the hood shape. This was to allow easier access to Chevy distributors, and it is generally understood that this was offered in the Type 3 and Type 4 models. The literature I have seen would seem to date this option back much earlier than that. I wouldn't be surprised if his car was one of the very early Arntz Cobras before Steve realised he didn't need to put 400lbs of fiberglass into every car!